The backplane issue seems to be a common problem, my 2 year old TVS-873e suddenly has 3 disks (3/4/5) disappearing, especially during boot time.
Several reboot attempts finally brought the disks back online, and a 24 hour (or so) rebuild made everything great again.
3 weeks later disks 3, 4 and 5 decided to disappear again while creating a Storage for QVR Pro.
Kinda sad, if you know that my 10 year old (or more) TS869Pro keeps humming away 24/7 since the day I bought it (it runs a mirror of the TVS873e).

I've been in contact with QNAP support, and yes they are VERY slow in responding. They did confirm the backplane issue, however my QNAP has been out of warranty since June (SOL).
They suggested a RMA, charging me $580 + shipping - getting dangerously close to buying a new one.

I'm very comfortable with taking the QNAP apart and doing some soldering, but some instructions would be helpful.

I understand more people are asking for this, so Herr Kommisar, without bothering you too much, would you be able to send me instructions as well (or anyone else for that matter)?
My contact info: hans at luijten dot net.

I just found out some interesting that may or may not be helpful to others;
I did get these unexpected "Disk Failed" and "Disk Unplugged" errors as well, as described in my previous post.

As I was tinkering with my QNAP (TVS-873), I opened the case and noticed a more stable environment pretty quickly after that (the array was actually booting just fine).

Adding an additional (very big) fan on top of the open case, actually made all components drop significantly in temperature (see table below).
As of that moment, my QNAP is running very stable, and I'm not longer seeing these errors.
Removing the fan however, made the array less stable again within a few minutes, and closing the case brought me back to the disappearing disks.

Now the "normal - closed case" temperatures and "extra cooled - open case" are in a very normal range, I would think anyway.

Even QNAP support confirms that these temperatures are quite normal, either way.
The PSU however becomes pretty hot.

Also note that I have a old TS869Pro running, with the exact same brand/model disks, and that one shows a higher CPU temp (different CPU, so no surprise there) at 52 C, and the disks are in the 35 - 38 C range (closed enclosure, no extra cooling) and runs just fine.

Since QNAP will charge an absurd amount of money for it's RMA services, I'm wondering if an extra fan could an alternative the solution.
Any advice/insight is most welcome.

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To summarize, the issue is HW related. At this stage there is no SW/FW fix. If your "Disk Unplugged", your NAS is on warranty and you see symptoms described above - contact QNAP. If it is not possible just follow the provided clues.

Thanks for posting this! I had a 473 die and RMA'ed, now a year and a bit later it is showing the same symptoms (last time they just swapped the backplane). I believe, following your clues, that I understand what to do but would appreciate a quick message if you are still watching this post to confirm I am on the right track / have identified the correct part. Or if anyone else has the details, please email me.

I curious as to why people are wanting tech advice from an email rather than just asking to post the fix in the forums for everyone to see. Asking for a guide for a fix via email is going to solve what for everyone else?